1) Field of the Invention
The present invention relates to a transfer device and an image forming apparatus that superimpose a plurality of visible images on a surface of a belt member, which is moved endlessly while being spanned by spanning members, to form a multi-superimposed image.
2) Description of the Related Art
In conventional image forming apparatuses, a plurality of monochrome toner images, which are visible images, are superimposed on an endlessly moving belt member (a belt surface, a sheet held thereon, or the like) to form a multi-color toner image, which is a multi-superimposed image. In such an image forming apparatus, a transfer system or a direct recording system is adopted as a unit that superimposes the monochrome toner images on the belt member.
Japanese Patent Application Laid-Open No. 2001-318538 discloses an image forming apparatus in which monochrome toner images are superimposed using the image superimposing unit adopting the transfer system, namely, a transfer device. In this image forming apparatus, monochrome toner images with colors different from one another are formed on a plurality of photoconductors placed opposite an intermediate transfer belt, which is a belt member, by respective electro-photograhic processes. The monochrome toner images are then sequentially superimposed and transferred onto the endlessly moving intermediate transfer belt.
Japanese Patent Application Laid-Open No. 2000-94734 discloses an image forming apparatus in which monochrome toner images are superimposed using the image superimposing unit adopting the direct recording system. In this image forming apparatus, toner images are formed without using such an electro-photograhic process; instead, electrostatic latent images are developed using the toner. Specifically, the image forming apparatus includes a plurality of image forming units, each of which can directly record a toner image on an intermediate recording belt serving as a belt member, by spattering toner group in a dotting manner through any of plural holes formed in an electronic substrate. Monochrome toner images of different colors are sequentially superimposed on one another, and are thereby recorded to an endlessly moving intermediate transfer belt at respective positions opposite the image forming units.
In each of the above image forming apparatuses, an endless moving velocity of the belt member depends on a circumferential velocity of a driving roller, which is rotationally driven while holding a spanned belt. That is, when a diameter of the driving roller changes due to an environmental fluctuation such as temperature or moisture, the circumferential velocity of the driving roller also changes accordingly. Further, the endless moving velocity of the belt member also changes depending on the change in the circumferential velocity of the driving roller. Thus, when the endless moving velocity of the belt member changes in this manner, the position where the respective color toner images are superimposed deviates. For example, if the endless moving velocity is stabilized at a velocity faster than an ideal value (uniform movement), when formation of respective color toner images starts at the same timing as the ideal value, toner images are position-deviated according to a sequential superimposing order thereof toward further downstream side in a moving direction of the belt. Superimposing deviation occurs due to the positional deviation. Alternately, for example, if the endless moving velocity is stabilized at a velocity slower than the ideal value (uniform movement), when formation of respective color tone images starts at the same timing as the ideal value, toner images are position-deviated according to a sequential superimposing order thereof toward further upstream side in the moving direction of the belt. In particular, when a roller, covered on the surface with elastic material such as rubber, is used as the driving roller for suppressing slippage between the driving roller and the belt member, the superimposing deviation may occur easily because thickness of the elastic material easily changes according to environmental fluctuation.
On the other hand, Japanese Patent Application Laid-Open No. 2002-287459 discloses an image forming apparatus that, based on pitch deviation of reference toner images for respective colors transferred on an intermediate transfer belt, periodically performs deviation correcting control for correcting light-writing timings to respective photoconductors. In the deviation correcting control, reference toner images for respective colors are theoretically formed on respective photoconductors at timings at which respective color toner images can be arranged and transferred on the surface of the intermediate transfer belt at equal intervals. The reference toner images for respective colors actually transferred on the surface of the intermediate transfer belt are detected by an optical sensor, and intervals of the reference toner images for respective colors are captured based on the detected time intervals. When the reference toner images for respective colors are not formed at equal intervals due to a velocity change of the intermediate transfer belt, light-writing timings to the respective photoconductors are corrected such that the reference toner images for respective colors are formed at equal intervals. Executing such a deviation correcting control periodically suppresses the superimposing deviation among respective color toner images occurring due to the diameter change of the driving roller according to an environmental fluctuation.
However, to suppress superimposing deviation among respective color toner images due to the diameter change of the driving roller according to the environmental fluctuation based on the deviation correcting control, the deviation correcting control needs to be performed quite frequently, for example, every time the environmental fluctuations exceed a threshold. This results in an increase in a frequency of the user rejecting a print start instruction, which is inconvenient for the user.